No abstract

The impact of extreme low flows on the water quality of the Lower Murray River and Lower Lakes (Alexandrina and Albert) in South Australia was assessed by comparing water quality from five sites during an extreme low flow period (March 2007-November 2009 and a preceding reference period (March 2003-November 2005. Significant increases in salinity, total nitrogen, total phosphorus, chlorophyll a and turbidity were observed in the Lower Lakes during the low flow period. Consequently, water quality guidelines for the protection of aquatic ecosystems were greatly exceeded. Principal Component Analysis, empirical and mass balance model calculations suggested these changes could be attributed primarily to the lack of flushing resulting in concentration of dissolved and suspended material in the lakes, and increased sediment resuspension as the lakes became shallower. The river sites also showed significant but more minor salinity increases during the extreme low flow period, but nutrient and turbidity concentrations decreased. The most plausible reasons for these changes were decreased catchment inputs and increased influence of saline groundwater inputs. The results highlight the vulnerability of arid and semi-arid lake systems to reduced flow conditions as a result of climatic changes and/or water management decisions.

Freshwater macrophytes may increase sediment redox potential and the affinity of sediments for phosphorus through radial oxygen loss from their below-ground biomass. This study demonstrated that the ability to alter sediment redox potential differs between macrophytes, according to their capacity to transport oxygen. Of the emergent macrophytes, Typha domingensis increased sediment redox potential (218 mV above bare sediment) to a greater extent than Bolboschoenus caldwellii (41 mV above bare sediment). However, the inhibition of convective flow in T. domingensis reduced its oxidizing ability by 78 mV. In contrast, Potamogeton crispus, a submerged macrophyte, had no influence on sediment redox potential. The presence of T. domingensis also increased phosphorus uptake from the water column by 0.88 mg P m−2 day−1, above that of bare sediments. In addition, inundation predictably decreased sediment redox potential from 175 mV to −176 mV over a 42-day period. Similarly, the addition of cellulose (10 mg L−1) decreased sediment redox potential by 42 mV. Consequently, deposition of organic debris may counteract the oxidizing effects of macrophytes that have a limited capacity to transport oxygen, such as P. crispus. Results suggest that macrophytes play an important role in facilitating the restoration of freshwater systems.

In streams, coarse particulate organic matter (CPOM) acts as a substrate for microbial activity, which promotes nutrient retention. However, in urban areas, increased peak flows within streams lead to decreased retention of CPOM. The aim of this study was to investigate whether the reintroduction of CPOM, in the form of leaf litter, into a degraded urban stream would increase biofilm activity and phosphorus retention, two ecosystem functions that reflect the integrity of the ecosystem. Stream metabolism and nutrient retention were assessed in treated (T) and control (C) channels of the Torrens River Catchment, South Australia, before and after CPOM addition. Gross primary production and community respiration (CR) were measured as oxygen production and consumption within benthic chambers. Phosphorus retention was measured through a series of short-term filterable reactive phosphorus (FRP) addition experiments. Before CPOM addition, there were no differences in CR, but C retained 6.8% more FRP than T. After CPOM addition, CR was greater in T than in C (572 and 276 mg O 2AE m 22 AEday 21 , respectively), and T retained 7.7% more FRP than C. The increase in FRP retention in T compared to C was attributed to phosphorus limitation of the CPOM and increased demand for phosphorus of the attached microbial heterotrophic community. The reintroduction of CPOM into degraded streams will be an important step in the restoration of stream metabolism and nutrient retention. Maintenance of CPOM may be achieved through restoration of riparian vegetation, a reduction in the increased peak flows, and rehabilitation of stream morphology.

Degraded streams have been shown to retain fewer nutrients than un-modified streams. The aim of this project was to investigate the relative importance of abiotic and biotic pathways of phosphorus uptake by epilithic communities in un-modified and modified streams. This was investigated through a series of filterable reactive phosphorus (FRP)-uptake experiments in two streams of the Torrens River Catchment, South Australia. Total benthic FRP uptake was assessed as the loss of FRP to unsterilized epilithic communities (k T ), abiotic uptake was the loss to sterilized epilithic communities (k A ) and biotic uptake (k B ) was the difference between total and abiotic uptake. It was hypothesized that un-modified reaches would have higher k A and k B than degraded and engineered reaches. Overall, k T , k A and k B were greatest in un-modified reaches, but this pattern was not consistent across seasons. k T and k B were greatest in the un-modified reaches in autumn-winter and late spring, but not in winterspring. Differences in k B were best explained by phosphorus availability in the water column and the period of continuous flow. k A was greatest in the un-modified reaches in autumn-winter, greatest in the degraded reaches in winter-spring, but similar in the un-modified reaches and degraded reaches in late spring. k A was most dependent upon the background FRP concentration, but also the attached organic matter in the un-modified reaches. The project demonstrated that several impacts of changes in land-use can alter the affinity of biotic and abiotic processes for phosphorus, which will have implications for in-stream nutrient availability and downstream ecosystems.